1. Field of the Invention
The present invention relates to a phase-locked circuit for outputting a signal, wherein a phase thereof is locked to an input signal.
2. Description of the Related Art
When using a phase-locked loop (PLL), for example as shown in FIG. 31, from a signal buried in an interfering wave, an output locked (synchronized) to a phase of the signal can be taken out. In this case, even if a frequency of the signal is changed by a Doppler-shift, etc., the PLL can follow the frequency change, and this is a different point from a mere filter.
Also, the phase-locked loop is effectively used also in the case of recovering a continuous signal shown in FIG. 32B from signals sent in a burst state, for example as shown in FIG. 32A. This is used for reproducing a color carrier wave, for example, in analog TV broadcast in the NTSC method, etc.
The phase-locked loop is also widely used for frequency synthesis. FIG. 33 is a basic block diagram of a frequency synthesis circuit (frequency synthesizer).
In this circuit, an oscillation frequency of a VCO (voltage controlled oscillation circuit) 4 is controlled, so that a reference frequency multiplied by 1/N “freq/N” and an output frequency fOUT multiplied by 1/M “freq/M” become equal by obtaining a phase difference Δφ of the both by a phase comparison band 2 via a LPF 3. As a result, the output frequency fOUT becomes “M×fREF/N”. By changing a division ratio M, it becomes possible to adjust the output frequency fOUT by resolution of fREF/N. The frequency synthesis circuit composed of a phase-locked loop is widely used for finely adjusting frequency in measuring devices and ratio communication apparatuses, etc.
A real number “a” is obtained by adding two conjugate complex numbers and dividing by 2, wherein the two conjugate complex numbers have positive and negative declinations having an equal size. In totally the same way, a real signal used in signal processing can be considered as a signal obtained by adding two complex signals having positive and negative frequencies.
In the case of performing processing relating to a frequency, such as frequency conversion processing and filter processing, on a real signal as above, two complex signals in positive and negative frequency domains may interfere.
For example, in a communication apparatus, etc., frequency conversion processing for taking out a signal of an intermediate frequency “ωIF=ωRF−ωLO” by multiplying a high frequency signal of an angle frequency “ωRF=2πfRF” and a local oscillation signal of an angle frequency ωLO=2πfLO” by the real signals is often performed. In this processing, as shown in FIG. 34, an image signal existing in an angle frequency ωimage=ωLO−ωIF also shifts to the intermediate frequency ωIF in the same way as the high frequency signal to be processed, so that it suffers from a disadvantage that it disturbs the communication. It is considered that this is because a complex signal of a negative angle frequency −ωimage included in the image signal shifts from a negative frequency domain to a positive frequency domain by being multiplied with a complex signal of a positive angle frequency ωLO included in the local oscillation signal to bring a frequency shift of “−ωimage+ωLO”.
In a phase-locked loop, only a real signal is capable of locking a phase, and positive and negative frequency components cannot be distinguished for use. Therefore, the interference of positive and negative frequency components cannot be fundamentally prevented and it suffers form disadvantages of causing complication of the circuit and a decline of quality of an output signal, etc.